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Wednesday, November 17, 2010

Photo Quiz

Do you know the species pictured here? Are you sure? Can you tell what age? Can you find it in your field guide?

Certain birds can be highly variable in their plumage appearance. An individual may not match exactly the illustration in the field guide. Most field guides show an "average" plumage; some show certain variations, but not all. The plumage above does not appear exactly in any of my field guides.

This is obviously a bird of prey. It has large, broad wings with "fingers" formed by emarginated (notched) primaries. Large yellow feet with sharp black talons, and a short curved bill complete the raptor characters visible.

The mostly black plumage with white base to the tail probably has most people immediately guessing eagle--either Golden or Bald. But which one?

If we only had a view of the upperside....

Well, we do. And it is immediately below. However, if you want to stop here and get your field guide out and make a guess, this is the place to do it. Because immediately after the photo below I will start giving the answer...

[Microsoft Windows tip: Right-click on each photo and "open in a new window" to get the larger image. Then you won't have to wait so long to reload the blog page as you do when you left-click and hit the "back" button to return.]

So, is it Bald Eagle or Golden Eagle? And what age? Why do we care about the age? We get more clues about the ID if we can determine age.

Let's start with what it is not, shall we?

It is not a juvenile Golden Eagle. Such a bird would have a brighter and cleaner white base to the tail and big white wing patches.

It is not an adult Golden Eagle. They have darker tails and less white on the face.

In fact, the white on the face and yellow on the bill eliminate any age of Golden Eagle. It must be a Bald Eagle. But what age?

The dusky banded tail, dark underparts, and dark back looks a lot like the juvenile Bald Eagle illustrated in Sibley's guide. However, the white on the face and yellow bill are wrong for a juvenile.

Second and third year birds have white wing linings and belly and a white triangle on the back. That doesn't fit. That's odd. And fourth year birds reach adult plumage with white head and tail.

I don't see any plumage age that matches the quiz bird.

Let's try the National Geographic Guide and... oh my God! It looks just like the juvenile White-tailed Eagle!

Wait! Slow down just a minute. While it is possible that I've actually found a rare bird--in fact, I have found several rare birds over the years--it is more likely this is an unfamiliar plumage of a common bird. After all, White-tailed Eagle from Eurasia is not just a rare bird in North America, it is extremely rare with only a few records from Alaska (mostly the Aleutians) and a record or two from the East Coast.

Let's age this bird more accurately to help with the ID.

Juveniles of many species, including eagles have more pointed feathers than older birds. Look at the wings--the primaries and secondaries on the trailing edge of the wing. The trailing edge is rather smooth and even. If this bird was a juvenile each feather would be obviously pointed, the trailing edge would appear saw-toothed.

Unlike smaller songbirds, the wings are so long on eagles that not all the flight feathers are replaced every year. It takes at least 2 years to replace the flight feathers. During this time the longer and more pointed juvenile feathers would contrast obviously with shorter and broader replacement feathers. There would be obvious "steps" (molt limits) every 3-5 feathers on the trailing edge. Such is not the case in this quiz bird.

Based on the state of molt on the wings (none visible) and no juvenile feathers, this bird must be at least 2-1/2 years old. As discussed, 2 and 3 year old Bald Eagles have white wing linings and often a white belly (especially 2-year olds), and a white triangle on the back. This bird does not. It has a more adult-like blackish body.

Not all Bald Eagles reach full adult plumage in 4 years, some birds take 5 years. It is possible, then, that our quiz bird is 3-1/2 (assuming hatched in May, and it is now November), with extensive dark mottling remaining on the tail and head feathers. I do note some white spotting throughout the body and wings.

A paper on aging eagles by Mark McCoullough would seem to indicate the bird I saw was in Basic III plumage.

However, everything we've said for Bald Eagle is also true of its sister species, the White-tailed Eagle. How do we know this is not a 3-1/2 year old White-tailed Eagle?

There are 3 remaining features to separate Bald Eagle from White-tailed Eagle. Rarely does one have to resort to these "supporting" field marks. But in this case it seems necessary.

For comparison, look at this album of White-tailed Eagle photos from Lithuania (isn't the Internet wonderful?). Bird 7 is a juvenile, looking superficially like our quiz bird. However, note the saw-toothed trailing edge of the wing?

Remember that we said that raptors show emarginated primaries? Bald Eagle has notches in the outer 6 primaries, White-tailed Eagle shows 7. On my photos I can only clearly see 6 notched primaries. The 7th? Well, it's hard to tell. Nothing conclusive.

The tail of White-tailed Eagle is short and pointed. My dorsal (back-side) photo above does not show an obvious pointed tail. Perhaps still not conclusive, but this mark seems to favor Bald Eagle.

Finally, Bald Eagles have white undertail coverts and White-tailed Eagles have dark undertail coverts. My ventral (under-side) photo definitely shows extensive dark undertail coverts. There appears to be a couple white feathers on the bird's right-hand side of the undertail coverts--maybe. If we assume this is a 3-1/2 year old bird, when all Bald Eagles should have extensively white undertail coverts, this mark definitely favors White-tailed Eagle.

A mark not mentioned in my meager references on White-tailed Eagles is the upper tail coverts. The uppertail coverts are white on Bald Eagle and mostly dark on White-tailed Eagle. My dorsal photo above shows 2 white uppertail covert feathers over the middle 2 tail feathers. This is exactly like this photo of White-tailed Eagle. This mark favors White-tailed Eagle.

That's it then. I can go no further based on my knowledge and references. Here is a bird that doesn't really match any plumage that I can find of either Bald Eagle or White-tailed Eagle. I believe it is in Basic III plumage, 3-1/2 years old.

Such an inconclusive outcome may happen anytime when we're bird watching. Sometimes birds are too far away, in bad light, or not seen long enough for us to be sure. In such a case it is best to let it go. Odds favor a common bird over a rare bird by, literally, millions to one. The odds are still the same even when photographed. And sometimes photographs--even fairly good ones--aren't diagnostic.

Unless someone can prove otherwise, with additional knowledge and experience I don't have, I have to go with Bald Eagle. But,... okay, take my own advice and let it go....

1 comment:

It appears to me that the leggings are obscuring the true nature of the undertail coverts and they're sufficiently white (based on what I can see) for the age class. It's always a good practice to think outside the box, but I think this can be safely ID'd as IIIB Bald Eagle.

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